While designed for 3A maximum, it will function just as well at low currents. If applied below 200mA, the heatsink may be eliminated. If applied below 1A, D3 may be reduced in size.

Nominal vs. actual charging voltage

When the voltage is stated (e.g. 12V), this is a reference to the nominal battery voltage (voltage in name only). Actual battery voltage ranges from 10.5V (fully discharged) to (14V fully charged).

Float charge voltage vs. full charge voltage in lead-acid batteries

The float charge voltage is the charging voltage that may remain connected long term—this is approximately 7V for 6V batteries and 14V for 12V batteries—actual manufacturer’s recommendation may vary somewhat, so it may be helpful to check the actual specifications. For faster charging, the voltage may be set slightly higher (e.g. 7.4V for 6V batteries or 14.5V for 12V batteries)—this charges the battery more rapidly, but requires that the control be intelligent enough to reduce the voltage to the float charge level after charging is complete. Most charge controls (like this one) simply charge at the float charge voltage—all automotive electrical systems do this.

Normally, solar panels are designed for a specific battery voltage applications. For 12V applications, the solar panel open circuit voltage is generally 18 to 20V. Similarly, for 6V applications, the solar panel voltage open circuit voltage is generally 9 to 10V. Since the 9 to 10V panels are relatively uncommon, it is not unusual to use 18 to 20V panels for charging 6V batteries. However, in this case the power dissipation of the series regulator transistor is multiplied by a factor of approximately 5. To prevent thermal destruction of Q1, the current rating in such cases must be derated from 3A to 1A.

A potential work-around is the addition of a 3Ω, 25W resistor in series with the solar panel—this reduces the control input voltage thus maintaining the 3A current rating.

The characteristics of a linear series regulator

The dissipated power is simply the voltage drop times the current. When there is current, but little voltage, the power dissipation is low—when there is voltage, but little current, the power dissipation is also low—when both current and voltage are present simultaneously, there is substantial power. Such is the nature of a linear regulator. The power is maximum when the battery starts to “top off” at the set voltage.

Dropout Voltage

The input voltage exceeds the input voltage by 0.7V when charging at the maximum rate—the lower, the better. Low Dropout Voltage (LDO) is the catch phrase for anything under approximately 2V. This is an important detail for 6V systems—for 12V systems, it is not generally a big issue.

Current Limiting

Current limiting is provided by the solar panel—it is not a commonly understood fact that the solar panel tends to be a constant current device. For this reason, a solar panel can withstand a short circuit. Therefore, the control itself does not require a current limiting feature.

Voltage Reference

The Texas Instrument TL431 is an inexpensive programmable shunt regulator. In this circuit, it is applied at the minimum voltage (2.5V) and does not require the usual voltage divider resistors. The TL432 is very similar, but the pin-out is different. Both are available in a variety of package outlines, but only the TL431 is available in the popular TO-92 package.

Circuit Operation

R1 biases D1, the voltage reference diode. The 2.5V reference from D1 is compared with voltage feedback from the resistor divider. The op amp does all within its power to keep these two voltages identical. The ratio of R3 /R2 controls the proportional gain, and C1 is a compensation capacitor that blocks DC feedback, but responds to changes in output signal thus maintaining stability (prevents oscillation). Zener D2 prevents overvoltage at the gate of Q1—R4 limits op amp output current when D2 is conducting. C1 is the positive rail bypass capacitor. D3 prevents battery voltage from appearing across the solar panel and prevents unnecessary battery discharge when the solar cells are not generating power.

When the feedback voltage from the wiper of R6 drops below 2.5V, the output of U1A moves in the negative direction thus turning Q1 on. The increased current out of Q1 causes the battery voltage to increase and increases the voltage at the wiper of R6 until it is equal to the reference voltage.

It may seem like a waste to use a dual op amp when only a single is required, but the LM358 remains the least expensive and most available device. It also has an undocumented feature that provides reverse battery connection. When the battery voltage is reverse, the non-inverting input of U1 is driven below the negative rail (common)—when this happens, the output of the op amp swings to the positive rail thus turning off Q1 and protecting the circuit against this potentially damaging condition. While this ‘malfunction’ is perhaps well known in the engineering community, the application of this as a circuit trick is new to the world.

Photos

For the future

Reduced intelligence MPPT charge control dispenses with the microcontroller